Short resonated antenna with step down transformer to improve resonant gain

ABSTRACT

A RECEIVER HAVING A SHORT RESONATED ANTENNA AND A SHORT LENGTH OF TRANSMISSION LINE BETWEEN THE ANTENNA AND THE FIRST AMPLIFIER STAGE IN WHICH HIGH RESONANT GAIN EFFICIENCY IS ACCOMPLISHED WITH THE AID OF A SUITABLY ARRANGED INDIRECTLY TUNED ANTENNA TRANSFORMER.

United States Patent [72] inventors of XX. [2!] Appl. No. 649,748 [22] Filed June 28, I967 [45] Patented June 28, I971 Volkers Research Inc. Port Washington, N.Y.

[73] Assignee [32] Priority June 29, I966 [33] Great Britain I l 29152/66 [54] SHORT RESONATED ANTENNA WITH STEP DOWN TRANSFORMER TO IMPROVE RESONANT 5| Int.Cl HOIq9/00 so FieldofSearch 343/745. 830,860,861,850

[ 56] References Cited UNITED STATES PATENTS 2,207,246 7/1940 Fischer .r 343/745 FOREIGN PATENTS 515,710 I2/l939 Great Britain 343/860 Primary Examiner-Eli Lieberman Attorney-Darby & Darby ABSTRACT: A receiver having a short resonated antenna and f 0D a short length of transmission line between the antenna and Chums l rawmg the first amplifier stage in which high resonant gain efficiency [52] U.S. Cl 343/745, is accomplished with the aid of a suitably arranged indirectly 343/861 tuned antenna transformer.

1 4-3) ll 3 23 f H 27 29 PATENTEMuueauen $588,902

* sum 1 nr 3 E FIG].

A resonated antcrina in an antenna which makes a major reactive contribution to a passive tuned input circuit. A resonated antenna it ffers from a resonant, or self-rewnating antenna, which, bylii irtue of its physical dimensions, namely precise multiples ofa quarter wave length, is capable of resonant operation without the aid of outside capacitive or inductive reactors. The rsonated antenna, on the other hand is either too long or too short for resonant operation without external reactive assistance.

The radiation capacity of an antenna is that portion of total capacity which is linked with space and contributes signal voltage, current and energy in reception. On the other hand ground capacity, which is not linked with space, does not contribute signal voltagel current or energy, and shunts the radia tion capacity, thereby reducing its effective signal contribution. Radiation capacity is connected in series with radiation resistance.

Resonant gain is the voltage gain of a resonant circuit at its resonant frequency. For the purpose of this invention the voltage gain is measuredlat the resonated antenna, with reference to the E-field in space. The resonant gain efficiency of a capacitive antenna depends on the ratio of radiation capacity to the total capacity of the tuned circuit.

Indirect tuning is a tuning method according to which neither the inductor nor the capacitor of a tuned circuit is varied directly, as for example, by the motion of ferrite slugs, or the rotation or compression of capacitor plates. Instead, a second inductor or capacitor is varied, and its variations are linked with the inductor or capacitor of the tuned circuit. Such linkage may be provided by a transformer, by a transformer-action of coils performing other, additional functions, or by other means well known to those skilled in the art.

Resonated subquarter-wave antennas are extremely efficient receiving antennas. Such antennas have low radiation resistance and hence do not create appreciable attenuation in the passive tuning circuits which couple the antenna to the first amplification stage of the receiver. Excellent signal-tonoise ratios can therefore usually be obtained at the input terminals of the first amplifier stage of the receiver, particularly since considerable freedom of design can usually be provided with regard to source impedance selection for the amplifier input terminals. Optimum signal-tonoisc ratios at these terminals calls for a specific, optimum signal source impedance, which is usually achievable through proper selection of tuning capacity and tuning inductance. Capacity selection dictates inductance selection which, in turn, determines the amount of untuned coil losses in ohms, which, in turn, influence tuned impedance losses in ohms, depending on coil-Q and resonant gain.

Thus a resonated, subquarter-wave antenna compares favorably with a self-resonant, quarter or half wave monopole or dipole antenna as long as its resonant gain efficiency, as defined above, is acceptable. Whether or not resonant gain efficiency is acceptable depends, however, on the coupling link between the antenna and the receiver which is normally a coaxial cable or shielded wire. Such a coaxial cable, even if substantially shorter than a quarter wave length, may add so much nonsignal-contributing capacity to the resonated antenna circuit that resonant gain efficiency is drastically reduced.

For example, in a conventional automobile radio receiver, having a vertical monopole or whip antenna or 6 feet long and having an effective height of approximately 2% to 3 feet, the typical total antenna capacity may be in the neighborhood of to pf., and the typical radiation capacity may be less than half of the total capacity when referred to the physical height of the antenna and more than half of the total capacity when referred to the effective height. The customary coaxial cable connecting the antenna to the receiver may be from 4 to 12 feet long depending on the location of the antenna, the size of the automobile, etc. Depending on the length and type of cable, the cable capacity may vary between pf. and I00 pf.,

including end-fittings. The total tuning capacity would therefore have to be at least (it) to pf, allowing for trimming capacitors and including the total antenna capacity.

Under the most favorable circumstances, using the shortest and best antenna cable (30 pf. for cable, approximately 25 pf. total aerial, 5 pf. for trimmer, total tuning capacity 60 pf.) resonant gain efficiency would be only approximately 20 percent, assuming that the signal contributing antenna radiation capacity is about 12 pf. Under the least favorable circumstances (IOO pf. for cable, 25 pf. again for aerial, 5 pf. for trimmer, total tuning capacity I30 pf.) resonant gain efficiency would be on the order of approximately 9 percent. In fact, the resonant gain efficiencics of automobile radio receivers in conventional use, were found to be generally closer to 9 percent than to 20 percent, the latter being a rare exception. Other communications receivers having longer transmission lines may have resonant gain efficiencies as low as 5, 3, 2 or even I percent.

It is therefore an object of this invention to provide a radio receiver having an improved resonant gain efficiency.

It is another object of this invention to provide a radio receiver having an improved absolute sensitivity which is defined as the sensitivity ofa receiving system based on signal to-noisc ratio only. Absolute sensitivity disregards total gain under the assumption that amplifier noise is dominated by the first stage of amplification, and that excessive gain created by an increase of passive resonant gain can be discarded, if desired, and that reduced gain, created by a reduction of passive resonant gain can be brought back up, if desired, by increased post-amplification which does not contribute to am plifier noise.

The increased absolute sensitivity of'a receiver can be used for improved performance, such as better signalto-noise ratio, or for cost reduction through elimination of amplified tion stages if the original signal-to-noise ratio is satisfactory, or for antenna miniaturization. The latter is of particular interest wherever antenna size is considered a nuisance as for example in aviation and in automotive applications. If increased absolute sensitivity is used to permit antenna miniaturization, heavy capacitive termination can be employed to increase ef' fective antenna height. This increase in effective antenna height can then be exploited by accepting the improved performance, or by trading the improved performance off against further miniaturiration.

The above and other objects and advantages of the present invention will be apparent to those skilled in the art from the following detailed description and accompanying drawings which set forth, by way of example, the principles of the present invention and the preferred modes contemplated for applying those principles.

In the drawings:

FIG. 1 is a schematic representation of a receiver including an antenna and a short transmission line showing the capacitive linkage between the antenna and space and the antenna and ground.

FIG. 2 is a schematic diagram of a receiver in accordance with the present invention.

FIG. 3a is a side elevation view in cross section of an antcnna transformer suitable for use in a receiver according to the present invention.

FIG. 3b is a cross section taken along the line b-b of the transformer shown in H0. 30.

FIG. ll is a side elevation view in a cross section of a modified antenna transformer suitable for use in a receiver according to the present invention.

FIGS. 5-9 are schematic diagrams of alternative receivers embodying the present invention.

Before referring to the detailed description of the drawings describing these illustrations in detail it should be emphasized that the present invention does not restin simple impedance conversions, such as commonly performed by ordinary antenna transformers. While such customary impedance conversions aim at maximum power transfer from the antenna into no input circuitry, the object of this invention is to create as large a voltage as possible, through passive resonance and thereby to reduce the influence of electronic amplifier noise as a contributor to total noise.

Referring to FIG. I the vertical monopole antenna I which may be a conventional whip antenna is flush-mounted at 2 within the essentially horizontal earth reference surface 3, such as, for example, a section of the body contour of a motor car. A coaxial cable 4 connects the antenna I in parallel with tuning coil or inductor 6 and tuning capacitor 7, either of which might serve as the frequency selector of the system while the other would serve as the trimmer for alignment purposes. Tuning frequency is determined not only by inductor 6 and capacitor 7, but also by four additional shunting capacitics, namely, the radiation capacity 8 of the antenna, the ground capacity 9 of the antenna, the ground capacity between the inner conductor it) of the cable and its outer shield II, and the incidental terminal capacity I2 at the tunerinput.

Resonant gain efficiency is determined by the ratio of radiation capacity 8, which is shown as connected to space signal source S, and the sum of all the above-described capacities. Among these, cable capacity 5 is usually found to be large, thus causing poor resonant gain cfficiency as explained above.

FIG. I also illustrates how the antenna radiation capacity is actually the equivalent of a large number of incremental capacities c c me which are shown to be individually linked with signal sources 5,, S ...S The voltages e,, e me of sources 5,, S S S, vary with height as shown on the left side of the antenna and represent horizontal equipotential planes within the vertical E-field surrounding the antenna. In the same manner the lumped ground capacity 9 of antenna 11 is actually the equivalent ofa large number of incremental capacities c c ...c that are linked with the reference earth plane 3.

In FIG. 2 a stepdown transformer 2t is connected between the antenna 22 and the variable inductor 23 of a transistor tuner 33. The ungrounded input terminal 24 of transistor 33 is coupled to tuning inductor 23 through a secondary winding 25 and blocking capacitor 26. The purpose of antenna stepdown transformer 21 is twofold. First, primary 27 of transformer 21 reflects the large cable capacity 28 and its two parallel capacities, the terminal shunting capacity 29 and the trimmer capacity 30, at considerably reduced magnitudes, the reduction being theoretically equal to the square of the transformers turn ratio between windings 27 and 3t. This raises resonant gain efficiency, which is proportional to the ratio of antenna radiation capacity to the sum of all capacities, referred to the antenna terminal. Second, transformer 21 permits variation of the effective inductance of primary coil 27, which can be considered the principal tuning inductance of the system, by being shunted by the remote variable tuning inductor 23. The inductance of the transformer primary coil 27 is much less burdened" by the unwanted high capacity 28 of the cable and other nonsignal-contributing capacities. The major shunting capacity of the transformer primary 27 is the relatively small total antenna capacity which consists of the antenna radiation capacity, which can be increased by capacitive termination 20, and the antenna ground capacity.

Major reduction of the total tuning capacity requires an increase in the tuning inductance. The use of a shunting inductor 23 for tuning purposes calls for a further increase of inductance of the main tuning inductor 27. The requirements for an increase in the inductance of the principle tuning coil 27 raise the problem of the increasing influence of distributed capacity across a coil which has an exceptionally large number of turns. While suitable winding techniques can be expected to keep turn-shunting capacities sufficiently low, to avoid harmful deterioration of Q, special techniques must be employed to minimize terminal to ground capacity of such a high-inductance coil and terminal capacity between the primary and secondary windings of the transformer. The latter of these has been found to be the most critical incidental design parameter which must be carefully minimized. Unless the terminal capacity 32 between primary 27 and secondary SI is kept low, the resonant gain improvement will be partially lost in spite of the substantial elimination of the influence of cable capacity 28 upon resonant gain efficiency.

In order to construct a suitable remotely tuned antenna transformer capable of covering a minimum frequency range of 1:3 one or more special techniques may be employed suchas for example partially ferrite partially nonfcrrite coil cores; noncylindrical coils, such as conical coils and auto-trans former operation.

FIG. 3a shows a side elevation view in cross section of an antenna transformer having a partially ferrite and partially air core. FIG. 3b shows a cross section taken along the line b-b of FIG. 3a. The transformer 40 has an inner primary 41!, which is wound directly on the core 42, which consists of ferrite material having a substantially larger permeability than air or vacuum. The transformer secondary 43 is held by two spacing rings, 44 and 45, made of low or unity permeability material which create a circular, cylindrical air space 52 between the windings M and 43, the purpose of which is the reduction of coil-to-coil capacity to values which are sufficiently low to make possible a high resonant gain efficiency. The inner coil 41 has a diameter 54 and the outer coil 43 a diameter 55. Assuming that the permeability of core 42 is unity and that loss of coupling due to stray flux is negligible, the coupling factor for the two coils 41 and 43 would be the ratio of their air core cross sections. In the antenna transformer shown in FIG. 3 this factor is approximately 40 percent. A coupling factor this low would make the transformer useless for remote tuning where a 1:3 minimum frequency range is required, calling for a l:9 inductance ratio and a minimum coupling factor of 91 percent to achieve such a large inductance ratio. However, since core 42 is, in reality, a ferrite core, the effective ratio of the cross sections of the cores ofthe two coils is substantially increased. When determining the core cross section of the outer coil, the portion of its cross section which is occupied by the ferrite must be multiplied by the permeability of the ferrite before combining it with the remaining cross section, which is the annular space between diameters 54 and 55. Thus, even though the ferrite occupies only 40 percent of the total core cross section, its effect is greater than the remaining 60 percent because of the lower permeability of air. The coupling factor F, is given by:

where A is the cross section linked with the inner coil 4t, A is the cross section linked with the outer coil 43, and a, is permeability of the ferrite core. If pcF-ZS, the coupling factor is approximately 94 percent rather than 40 percent and tuning range is increased by the square root of the improvement.

FIG. 4 is a side elevation in cross section of an antenna transformer with noncylindrical coils. Outer coil 43a is conically shaped. The left edge 46 of outer coil 43:: is a larger distance 47 from the inner coil M than the right edge 48 of outer coil 330 which is distance 49 from inner coil 41. The purpose of the conical coil arrangement is to provide extra distance 67 between the coils in the region where stray capacity between the coils most seriously affects resonant gain efficiency, which in this case is the region of the high end of the transformer primary 41 and/or secondary 43a. Accordingly, the left ends of both coils 4t and 43a correspond to the ends which are the farthest electrically removed from ground and from each other in the circuit diagrams ofFIGS. l and 59. It will be apparent to those skilled in the art that other noncylindrical coil arrangements will serve the intended purpose, such as, for example, a conical inner coil and a cylindrical outer core, or conical inner and outer coils.

It will be further apparent to those skilled in the art that the present invention is not limited to transformer coils having circular cross sections. For example, the transformer coils may be rectangular or square-shaped, etc. The shape of the transformer yoke may also be varied from the U-vihape shown in FIGS. 3 and d to ltUCh shapes as double yolteri, figurex of rotation, etc. The locationn of primary and secondary may also be reversed.

A third technique for increasing resonant gain efficiency and tuning range, which hart been found to be highly effective, especially when used in conjunction with one or both of the above described techniques, is autotransformer-action. HO. 5 shows a receiver similar to that shown in MG. l, except that primary and secondary of the antenna transformer are seriesconnected. In addition to the generally recognized wire-saving property of autotransformers, there is the advantage that the current flowing in tuning inductor 23 reduces the inductance of main tuning inductor 27a3iia, which is the primary of the antenna transformer, not only through transformer-action between sections 27a and 31a but also by shunted-divider-action. Even if no transformer-action occurred between sections 27a and 31a of the autotransformer, the shunting of its secondary section 3hr by variable tuning inductor 23 would create an inductance change in the total inductor 27a-3la. In fact the two effects are mutually additive and facilitate the achievement of the desired 1:3 tuning range without need for an unreasonably large inductance range in tuning inductor 2.3.

FIG. 6 shows a receiver similar to that of FIG. 5 except that the tuning inductor secondary 25 of Fifi. a has been relocated and transferred to the antenna transformer 21! a tertiary winding 25a which serves as an input winding to amplifier 33. Other components, common to both FIGS. carry identical nu merical designations.

FIG. 7 shows another receiver similar to that of FIG. 5 except that the tuning inductor secondary 25 of FIG. 5 has been split into two series-connected windings 25b and 25c. Winding 25b remains in the original location as a secondary winding to tuning inductor 23, while the other winding 25c is a tertiary to antenna transformer 21. Both windings 25b and 25c provide input signals to amplifier 33.

Preference for one of the three alternative circuits in H08. 5, 6 and 7 should depend on availability of magnetic flux in the cores of antenna transformer 21 and tuning inductor 23 at various points of the tuning range since the flux in either core is available as a voltage and power source for the signal to be delivered to the input of the transistor amplifier. If the antenna transformer 21l were an ideal transformer, with 100 percent coupling between its primary and secondary coils and no stray reactance, the flux in the transformer core 211 would steadily decrease and the flux in the core of the tuning inductor 23 would increase as inductance ofthe latter is lowered to tune in higher frequency signals. In fact, the antenna transformer 2i does not have perfect coupling and its stray reactance, as seen from the terminals of its secondary creates an increasing reactive voltage drop in the transformer 21 which reduces the increase of current flowing in the tuning inductor 23 which, in turn, holds back the theoretical increase of voltage across tuning inductor 23 as well as voltage and power generated in the secondary winding 25 of FIG. 5 or 25b of FIG. 7. Thus the circuit of FIG. s, using the tertiary winding 25a on the antenna transformer core 21 and no secondary on the tuning inductor 23 would appear to be a preferable signal source for the input of the amplifier 33. On the other hand, flux reduction in the primary 27a of the transformer due to increasing in current in the secondary Ella when the inductance of the tuning inductor 23 is reduced to increase the tuning frequency, tends to diminish the extraction of the amplifier input signal from the core 21 of the antenna transformer. The circuit in FIG. '7 provides a compromise in which energy is extracted from both the coreZl of the antenna transformer and the core of the tuning inductor 23. There are certain quantitative design features in the selection of locations for the winding, or windings, which deliver the signal to the input of the amplifier. These are described in the claims and their statement in the specification, in numerical details, would only create an unnecessary duplication.

The foregoing considerations are also applicable to the an tenna transformer circuit of FIG. 2 which does not use autotransl'ormcr action. l-lti. it shows a circuit similar to that of MG. 2, except that the secondary 25 of the tuning inductor in FIG. 2 has been relocated at the antenna transformer as tertiary winding 25a ofFlG. ii.

FIGv it shows another circuit similar to that of FIG. 2 in which the secondary 25 of the tuning inductor 23 has been split into two series-connected windings 25b and 25c with winding 21% remaining in the original location as a secondary to tuning inductor 23 and winding 25c as the tertiary to antenna transformer 2i.

Remote tuning by a variable shunting inductor which is coupled to the main tuning inductor by transformer action makes it necessary to provide comparatively tight coupling between the primary and secondary of the antenna transformer while minimizing stray capacities between the two windings. The means for accomplishing this have been described. ln addi tion, it is important to maintain a reasonably good 0 of the entire tuning system over its complete frequency range. In order to achieve this the antenna transformer itself should preferably be provided with a generous spare tuning range" because an excessively large inductance range must otherwise be available in the remote shunting tuning inductor, also because excessive losses are developed in both the antenna transformer and the tuning inductor.

The spare tuning range of antenna transformer is determined by shorting the secondary of the antenna transformer measuring the inductance of its primary and setting this in ductance in relation to the lowest effective tuning inductance required at the primary terminals when the system is tuned to its highest frequency. Then a second measurement of the inductance of the transformer primary is made while the secondary is open-circuited, and again the inductance is set in relation to the highest effective inductance required when the system is tuned to its lowest frequency. The first measurement establishes the extension of the spare tuning range beyond the upper frequency limit of the receiver and the second measurement determines the extension beyond the lower frequency limit. The ratio of the extended upper frequency limit to the extended lower frequency limit is then set in reference to ratio of the required upper tuning inductance limit to the required lower tuning inductance limit.

A minimum spare tuning inductance range of twice that of the required inductance range, or a minimum spare frequency range of /2 times the required frequency range is preferred in order to avoid losses within the system. Losses may be further reduced by a spare inductance range 3 times the required in ductance range or a spare /3 times the required frequency range.

The following example ofa circuit in accordance with FIG. 7 illustrates the importance of providing an adequate spare tuning range for the antenna transformer:

Transformer primary, upper section 270:20 mh.

Total primary (27a and 31a in series):22.5 mh.

Transformer secondary (31a) alone: 1.8 mh.

Tuning inductor (23) range: 25 mh. to 0.8 mh.

Primary inductance range, as controlled by tuning inductor:

l4 mh. to 1.5 mh.

lnductance range of primary, with secondary (31a) open and shorted: 22.5 mh. to 0.75 mh.

Low inductance spare range: 1.5 mh./0.75 mh.=2.

Upper inductance spare range: 22.5 mh./l4 mh.=l .6.

Spare tuning-inductance range of antenna transformer: 3.2

to l.

Tuning capacity referred to antenna but exclusive of anten na ground and radiation capacities: 3 pf.

Total capacity shunting transformer's secondary: approx.

I00 pf.

Antenna radiation capacity: 3.2 pf. (miniature antenna 2 inches high with 2 inch termination disc).

Resonant gain efficiency of nyiitem 3.2 pf/(B pf.+3.2

pf.)=5 l .6 percent.

Antenna gain 26 to 10 over I :3 frequency range.

It will be apparent that without the transformer arrangement of this invention that the name miniature antenna would show a resonant gain of approximately one-thirtieth, or 0.7 to 0.35, and that remnant gain efficiency would be as low as 1.7 percent,

While the principles of the present invention have been illuatrated by reference to a number of examples of iipecific receiver system employing varioms phase control devices, it will be appreciated by those skilled in the art that the prerient invention is not limited to those examples. it will further be apparent to those skilled in he art that other modifications and adaptions of the disclosed systems may he made without departing from the spirit and scope of the invention as set forth with particularity in the appended claims.

lclaim:

l. A receiver system comprising a resonated antenna; a variable reactive tuning device; a transmission line connected between said antenna and said variable reactive tuning device, the effective capacity of said transmission line being in shunt relationship with the radiation capacity of said antenna; and a stepdown transformer, said transformer including a magnetic core having a permeability greater than unity, an inner winding surrounding said magnetic core, an outer winding surrounding said inner winding, said outer winding being separated from said inner winding by a substantially annular space having a permeability substantially equal to unity so as to provide reduced capacitive coupling between said inner and outer windings; the primary winding of said transformer being connected to said antenna and the secondary winding of said transformer being connected to said transmission line so as to reduce the reflected capacity of said transmission line at said antenna and thereby increase the resonant gain efficiency ofthe receiver system.

2. The receiver system of claim I wherein the primary and secondary coils of said transformer are connected in parallel.

3. The receiver system ofclaim 1 wherein the radial dimension of said annular space is relatively greater at the end of said winding farthest from ground.

d. The receiver system of claim I wherein said variable reactive tuning device comprises a variable tuning inductor.

5. The receiver system ofclaim 4 wherein the maximum inductance of said variable tuning inductor is less than the open circuit inductance of the secondary winding of said transformer.

6, The receiver system of claim 4 wherein the spare tuning inductance range is at least twice the required tuning inductance range.

7. The receiver system of claim 6 wherein the spare tuning inductance range is at least 3 times the required tuning inductance range.

8 The receiver system of claim I wherein the spare frequency tuning range of the transformer is at leasu rltimes the required frequency tuning range.

9. The receiver system of claim 1 wherein the spare frequency tuning range of the transformer is at least\/ times the required frequency tuning range.

10. The receiver system of claim 1 wherein the total capacity to ground as seen by said antenna at the input to said transformer is less than 5 times the radiation capacity of said antenll. The receiver system of claim 1 wherein the total capacity to ground as seen by said antenna at the input to said transformer is less than the radiation capacity of said antenna. 

